Building Semantic Web Services Infrastructure for Digital Libraries

Transcription

1 Building Semantic Web Services Infrastructure for Digital Libraries Sebastian Ryszard Kruk, Adrian Mocan, Brahmananda Sapkota, Michal Zaremba DERI, National University of Ireland, Galway, Ireland Abstract. We have seen an increasing interest of applying Semantic Web Services to enhance functionality offered by existing Web Services standards. While the main stream of work still focuses on building the conceptual basis for Semantic Web Services, initiatives like WSMX attempt already to apply these efforts by addressing requirements of various business domains for automatic discovery, mediation and invocation of services over the network. In this paper we discuss a research that has been carried out to enable automatic interoperability between heterogeneous network of digital libraries enabling users to search across distributed network of libraries based on different metadata standards. We will present how the P2P backbone of WSMXs can support communication between existing library-to-library (L2L) networks. We will discuss the possible integration of WSMX with exists L2L protocols like DIENST, Open Archives Initiative (OAI) or Extensible Library Protocol (ELP). 1 Introduction In recent years access to the Internet has become a commodity. The growing number of users with access to high speed network connections is causing increased demand for high quality and accurate information. One of the sources of high quality information tend to be libraries. On the Internet, digital libraries are often islands of high quality and well organized information. Digital libraries not only make electronic information available, but also provide access to legacy information (e.g. old historic books, documents, magazines, etc.) which were previously only accessible to a restricted minority of privileged library users. This work is supported by the SFI (Science Foundation Ireland) under the DERI-Lion project, and by Gdansk University of Technology, and by the European Commission under the projects DIP, Knowledge Web, SEKT, SWWS, and Esperonto, and by the Vienna city government under the CoOperate program. The authors thank all members of the WSMX (cf. the JeromeDL (cf. and the MarcOnt Initiative (cf. working groups for fruitful discussions on this document.

2 To provide user with high quality searching features digital libraries tend to go beyond trivial implementations acting as brokers between users and physical library. They join peer-to-peer networks extending amount and quality of data, which can be provided to users. Digital libraries tries to supply readers with searching features that would span resources across the Internet. Similar solutions have been already implemented in classical libraries. The difference is that nowadays, user expects to get results immediately (or in reasonable time) instead of waiting weeks as it was in pre-internet times. As a number of digital libraries proliferates, it becomes more difficult (or even impossible) to locate and execute services that can perform search in many networks of heterogeneous libraries. Sufficiently rich, machinable-readable instances of messages exchanged between library systems would allow execution of queries across different peer-to-peer networks. Having instances of messages lifted to ontology level we would be able to mediate between heterogeneous data formats and query libraries afterwards. The mediation process could be run without human intervention, using different metadata standards than the one originally used by requesters. We believe that it make sense to apply Semantic Web Services to enhance functionality offered by existing digital libraries so to make the search across heterogeneous networks feasible. For that purpose we use Web Services Execution Environment (WSMX) attempting to enable automatic interoperability between islands of heterogeneous digital libraries. The user could then search across libraries using different metadata standards. We discuss the problem of adapting between heterogeneous metadata standards and Semantic Web Services platform, as well as mediating between various bibliographic ontologies. This paper is organized in the following manner. In section 2, we present digital libraries and selected metadata standards used for library-to-library communication. In section 3, we explain why semantics need to be included with existing Web Services, we give a background information about Web Services Modeling Ontology (WSMO) initiative and we introduce Web Services Execution Environment (WSMX). Section 4 presents how we propose to search across heterogeneous peer-to-peer networks of digital libraries using WSMX and Semantic Web Services. In section 5, we summarize some related work. And finally in section 6, we conclude our work and present future directions for this work. 2 Digital Libraries Digital libraries are facing challenges managing an every increasing amount of resources caused by the increase of investment in research and development as well as the trend to produce more and more written information. Several solutions to these challenges have been developed. Classic libraries are respected for the quality of service they provide. In order to reach this status of trust and reliability, digital libraries require effective information access facilities. But digital libraries are not restricted to conven-

3 tional means - e.g., information access can also benefit from the development of extensible browsing facilities based on the social connections between readers. 2.1 Library-to-Library communication A recent trend in digital libraries is to connect multiple digital libraries to federations. Each digital library, apart from delivering discovery and navigation features, provides the ability to search among other digital libraries systems. That way readers do not have to query every single library themselves. The request is forwarded to other digital libraries and presented to the user in a compact form. At present, different digital libraries can use different metadata standards. This also implies a variety of protocols available: Z39.50 is a session-oriented and stateful, network protocol[1]. The primary goal is to provide an international standard for network information search and retrieval. The protocol allows the user to access remote database records by specifying criteria to identify appropriate records, and then requesting the transmission of some or all of the identified records. DIENST is HTTP based network protocol[2]. The HTTP GET queries are handled by specialized set of services called Verbs. The answer from the service can be provided in one of the mime types: text/plain, text/html or text/xml. OAI-PMH the Open Archives Initiative Protocol for Metadata Harvesting[3] provides an application-independent interoperability framework based on metadata harvesting. The OAI-PMH defines two classes of participants: data providers and service providers. The OAI-PMH is based on HTTP requests and XML responses. The response utilizes DublinCore as metadata format. SDLIP The Simple Digital Library Interoperability Protocol[4]. The aim of the project was to simplify the implementations of distributed searching in digital libraries domains. The result documents were returned synchronously, or they were streamed from service to client as they become available. ELP The Extensible Library Protocol [5] build as an L2L extension for JeromeDL 1 library. It is based on Web Services (SOAP) and utilizes DublinCorebased ontology as a base metadata for describing queries and results. The list of protocols presented above is not complete and no final as well. No matter how many protocols will be introduced, there will always be a reason to create a new one. To provide more flexible way of connecting L2L networks based on different protocols we introduce Semantic Web Services infrastructure. 3 Web Services Execution Environment Cornerstone Web Services specifications have been developed to address problems of interoperability between distributed applications build by various ven- 1 JeromeDL - e-library with Semantics

4 dors. Existing Web Services technologies provide basic functionality for discovering (UDDI), describing interfaces (WSDL) and exchanging messages (SOAP) between heterogeneous, autonomous and distributed systems. These technologies enable service requesters to locate and execute services described by interfaces, hiding implementations and abstracting service from concrete platform or programming language. 3.1 Semantic Web Services In practical terms, existing Web Services technologies support operations limited to services where each operation remains an independent call over the network. But when we move from simplistic information lookup to complex interactions of systems build by various vendors, it occurs that current Web Services technologies remain inadequate to support definition of services, which are supposed to be processed automatically. The motivation behind the work on Semantic Web Services is to look ahead of existing efforts. Web Services standards do not provide any mechanism to specify how to include any additional semantic information which would enable processing them without human interaction. Next section introduces Web Services Modeling Ontology (WSMO) initiative built on existing efforts of standardizing of Web Services, aims to overcome limitations of existing standards enabling automatic service processing. 3.2 Web Services Modeling Ontology The Web Service Modeling Ontology (WSMO) [6] is a formal ontology for describing the various aspects related to Semantic Web Services. WSMO constitutes a backbone for the development of Web Service Modeling Language (WSML) [7] and Web Service Modeling Execution Environment (WSMX). Major objective of these initiatives is to define a coherent technology for Semantic Web Services. WSMO defines a complete set of modeling elements necessary for describing all of the aspects of Semantic Web Services. It aims at enriching syntactical Web Services with semantic mark-up to address weaknesses preventing non-human users from automatic discovery, composition and invocation of Web Services. The conceptual grounding of WSMO is based on the Web Service Modeling Framework (WSMF) [8], adhering to the principles of loose coupling and strong mediation. There are four main components in WSMO: Ontologies provide the formal explicit specification of shared conceptualization [9] - terminology and formal semantics to the information used by all other components. Goals are the high level description of concrete task expressed in ontology of service requester. They specify objectives that service requester have when consulting a Web Service.WSMO goals are described in terms of the desired information and world state that must result from the execution of a given service.

5 Web Services fulfills certain purpose and as that they represent the functional part, which must be semantically described in order to allow their semiautomated use. Capabilities of Web Services, which are part of their description, capture the functionality of a given service. Capabilities are modeled in terms of the preconditions and assumptions for the correct execution of the service and the postconditions and effects resulting from this execution. Mediators address a problem of heterogeneity between service requestors and providers - they are used as connectors in order to provide interoperability among the other components. The formal language for WSMO is WSML [7]. WSML defines a family of formal representation languages with its roots in Description Logics and Logic Programming. 3.3 Execution Environment for Semantic Web Services The work on WSMX, a multithreaded, event driven system, based on a microkernel design, addresses problems of discovery, mediation, invocation and interoperation of Semantic Web Services. System follows the design based on Service Oriented Architecture (SOA) paradigm. WSMX allows agents acting on behalf of a user to discover and execute Web Services enhanced with semantic metadata. Although existing implementation of the system still remains incomplete in terms of the required functionality, several components are already in place, and the system is capable of executing simple scenarios like the one required for interacting between heterogeneous digital libraries. 4 Connecting Different Protocols For the purpose of this research we have envisioned two scenarios (the complex and the simple one), where one digital library intend to query another heterogenous digital library. While the complex scenario is still under investigation as there is not enough functionality provided by WSMX components yet, the simple scenario is already feasible to be executed. In the more complex scenarios digital libraries may not know other digital libraries, which they want to query, so the appropriate library has to be first discovered, before query can be issued to it. Actually to make this solution scalable we never query single libraries but we issue queries only to entry points of peer-to-peer networks of libraries using the same meta-data standard. 4.1 WSMX for Digital Libraries WSMX hides all the heterogeneity data and protocol difference between network of digital libraries, so library issuing query perceives as it would be queering its own peer-to-peer network. WSMX must take care to resolve the request into representation, which it uses internally (into WSML) in order to discover Semantic Web Service and to execute the query in the other peer-to-peer network

6 of digital libraries. In a simpler scenario, which is currently feasible with existing set of components, requesting library have to know how to locate library which should be queried (so no discovery or selection is necessary to realize this scenario), but the protocol and data representation used by requesting library are different from the meta-descriptions used by library providing query results. Instance data, which is sent with request and reply must be lifted to ontology level and mediated before it is forwarded to their destinations (see Fig. 1). Fig. 1. L2L network 4.2 Searching in L2L networks Digital library should support communication with federated digital libraries by providing a communication infrastructure for a distributed network of independent digital libraries (L2L)[10, 5] similar to communication in a P2P network. The ELP[5] uses SOAP as a transport protocol. The solution is based on the fact that the searching process in JeromeDL, ecapsulates all information (the query object and the result object) in XML. The use of Web Services for building the P2P network of digital libraries will enable connecting JeromeDL to the ongoing projects like OCKHAM 2. The idea of the ELP is to allow communication in the heterogeneous environment of digital libraries. Each library has to know about at least one other 2

7 digital library, so it could connect to the L2L network. The minimal requirement imposed on the digital library is to support at least the DublinCore Metadata. If two digital libraries describe the resources with semantics, like JeromeDL system, the communication between them is automatically upgraded to the semantic description level. It allows to use the search algorithm with semantics in the L2L communication. Fig. 2. Searching in heterogenous L2L network with WSMX HyperCuP backbone To provide scalable solution for ELP on the field of mediation with other L2L standards we propose the architecture based on the P2P network of WSMXes. The WSMX instances are joined in the P2P network of HyperCuP architecture[11, 12]. Figure 2 presents the step-by-step scenario of communication in heterogenous L2L networks (see Fig. 1).

8 Posting a Query When user communicates with a digital library, like e.g. JeromeDLhis query is first processed by the local library search engine. Afterwords the query is being dispatched to other libraries confederated in the native L2L network, e.g. ELP-based. One of the WSMX instances plays a role of a peer in this L2L network, as it can communicate in the native protocol. Communication between WSMXes The query in the native protocol is translated by an adapter to the format understandable by the WSMX. Later on, the query in the WSML format is posted, according to the HyperCuP rules, accross the P2P backbone of WSMX network. Each of the WSMXs uses the adapter to lift the query to the ontology level required by the L2L protocol it plays a part of. The WSMX mediator solves the heterogenous problems that can appear at the semantic level. The adapter translates the query to the native protocol format in the destination L2L network. Figure 3 presents the transformation performed when a query is posted to the ELP-enabled libraryand which is going to be answered by a DIENST library. Accordingly, a set of symmetric transformations is applied to the result on its way back to the original system where the query was formulated. All these transformations are made in order to resolve the syntactical and semantic heterogenity problems that can appear when passing the query/result between libraries. These problems tackled by adapters and mediators are solved by WSMX systems. Note that two WSMX systems acting as a libraries in the native L2L network exchange only WSML instances, so transformations presented above do not take place entirely inside the same WSMX system. More details about these transformations can be found in following sections Gathering an Answer Each WSMX gathers the results from the L2L network it communicates with. The results are translated with appropriate adapter to WSMX native format. Each WSMX instance sends back the aswers to the one that has posted the query (quering WSMX ). The process can be done by a direct answer to the quering WSMX, when the messages are passed in the assynchonous manner. The quering WSMX translates the gathered results to the result format of the native protocol in its L2L network. The result set is sent directly to the library that initated the query, and presented to the user later on. 4.3 Adapting Syntax of Messages Different data representation format used by different systems introduces an integration problem. Emerging standards like XML and SOAP helped to solve these problems to some extent. However, they concentrate more on protocol level ignoring importance of data format in integration [13]. When two systems can not understand each other s data represented in their own format it is difficult, if not impossible, to make them communicate with each other. This situation mandates the use of some kinds of intermediary component that can facilitate communication between such heterogeneous systems.

9 Fig. 3. The role of adapters and mediators in a distributed query answering scenario We call this intermediary component an Adapter. Depending on the types, requirements, and complexities of the systems under integration, adapters can be placed in three different categories: application integration, infrastructure integration, and format integration adapters [14]. Format integration adapters are capable of transforming data represented in one format to another (see Fig. 4). Fig. 4. Format integration adapter Adapters address a problem occurring before the interoperability becomes an issue - namely they enable connecting external systems such as digital libraries to WSMX. In current design of system architecture, adapters remain outside of the system, but they have a critical role in overcoming data representation mismatches. Most of the existing systems is completely unaware of WSMO and WSML and they cannot communicate with WSMX using their own data

10 representations or their own protocols. On a conceptual level, an adapter transforms the format of a received message into the WSML compliant format, which can be next understood by WSMX. The process of transformation (adapting) uses mapping rules which are concerned purely with the syntactical mappings between messages formats while maintaining the semantics of these messages. These mapping rules are generated and specified in XML Stylesheet based on the XML Schema of the requesting system and the XML Schema of WSML. Using this XML Stylesheet, every data received on requesters format is then transformed to WSML format. We use XSLT [15] for this transformation(see Fig. 5). Fig. 5. Convertion from Digital Library to WSML format Same technique can be applied to transform data specified in WSML format into the format that can be understood by requester. The role of adapters in L2L communication As seen in the Figure 3, when a ELP digital library initiates a query it will be lifted up to WSML instances and forwarded to WSMX using the ELP2WSML adapter. After receiving this WSML instance of the query, WSMX either returns the results or forwards it to other WSMXs in the network. When the forwarded request is received, a terminal WSMX may wish to invoke DIENST digital library, for example. At this time, the WSML instance will be lowered down to DIENST query using WSML2DIENST adapter and forwarded to DIENST digital library. Likewise, when the response have to leave DIENST digital library it will be lifted to WSML instance using DIENST2WSML adapter and pushed to WSMX. Finally, when the result leaves the WSMX is processed by WSML2ELP adapter which then lowers the response from WSML instance to ELP result and forwards this lowered result to ELP digital library user.

11 4.4 Mediation between Heterogonous systems WSMX offers a default implementation for a Data Mediator able to reconciliate the heterogeneity problems existing at the data level [16]. This mediator acts on the semantic level, making use of the domain conceptualization (i.e. ontologies) used by different parties. It also enables intercommunications between them by transforming the exchanged data from terms of one ontology (source) to terms of the other ontology (target). It corresponds to the ontology to ontology mediator from WSMO specifications [6] and its scope is to provide instances transformations based on the similarities identified at the schema level of two ontologies. The data mediation process consists of two distinct phases: design-time phase and run-time phase. During design-time phase similarities between source and target ontologies have to be identified, and expressed in a form that can be saved in persistent storage (i.e. mappings) for further usage. The similarities are identified with a help of a domain expert, in a computer assisted manner. This phase corresponds to the schema matching and the mechanism meant to help the human user in his work with hybrid and composite matchers[17]. The run-time component uses the mapping defined during run-time phase and executes the mappings against the source instances in order to generate the corresponding instances in terms of the target ontology. This step is performed completely automatically without human user interaction and outputs a set of instances in terms of the target ontology. The set is ready to be sent to the target party. The role of mediators in L2L communication Figure 3 shows how mediation fits in the overall scenario presented in this paper and in which stages mediation is necessary. Please note that what we understand by mediation is the process of resolving heterogenity problems that appear at the semantic level (the syntactical mismatches are solved by adapters), and as a consequence, in this case, the mediation happens only between WSML instances. Following the example in Figure 3 the WSMX system used by ELP-based library gets from the adapters the query expressed as instances of a lifted ELP schema. No mediation will be required for this query since this WSMX doesn t know to which node the HyperCup infrastructure will route the request. The WSMX node that can process this query has to transform the WSML instances from terms of ELP lifted schema (source ontology) to terms of DIENST library lifted schema (target ontology). For being able to do this the second WSMX has to be aware of mappings, created during the design-time, between two ontologieswhen the mediated data is obtained it is forwarded to the adapters for lowering. For the results returned by the DIENST library the same set of operations have to be performed, but in a reversed order. Actually, the whole process is symmetric, and the result is mediated by the WSMX system used by ELPenabled L2L network.

12 5 Related work Beside WSMX there are other software tools providing support for execution of Semantic Web Services having their roots in OWL-S, Meteor-S and WSMO initiatives. There are also several commercial integration platforms capable of overcoming integration problems between heterogeneous systems. In this section we provide a short overview of them, evaluating their applicability in supporting digital libraries in exchanging information across heterogeneous networks. We also look into related work on digital libraries. IRS3 is a platform developed by Knowledge Media Institute at the Open University, capable to handle WSMO and OWL-S based Semantic Web Services [18]. In IRS3 design environment provider of the service creates WSMO based service description and publishes it against its service on the IRS3 server. Having service available, goal can be described and bound with existing Web Service using a mediator. Although this approach sounds quite limiting for our scenario (Web Services must be known to IRS3 server at the design time - differently than in our system when they are not known until run time), one can imagine IRS3 running as a node instead of WSMX node in our peer-to-peer network. An attempt has been already undertaken to bring interoperability between WSMX and IRS3 - currently two major Semantic Web Services Platforms [19]. Meteor-S builds on existing Web Services technologies providing a framework for Web Services composition and discovery [20]. Meteor-S is based on language predecessor of OWL-S, which is used by OWL-S. There is no comprehensive strategy for development of Meteor-S server, as it is in case of WSMX or IRS3 frameworks. Rather there are multiply efforts to address different aspects of Semantic Web Services. The main tool - MWSAF - provides ontology store, translator library and matcher library. Another tool Meteor-S WSDI is a peer-to-peer infrastructure for accessing and annotating multiple registries. Although some of these components could become an elements of our infrastructure enabling queering heterogeneous libraries, there is no single infrastructure for Meteor-S which could be used as peer-to-peer nodes to support scenarios as presented in our paper. OWL-S [21]is a competitive effort to WSMO initiative attempting to define ontology for Semantic Web Services. Similarly to Meteor-S there are multiply tools available, but there are no integrated strategy of development a complete infrastructure for execution of OWL-S Web Services. There is composer, matchmaker or editor, but run-time infrastructure capable to handle execution and coordination between all these components is not yet available. WSMX development team is going for testing purposes to accommodate some of OWL-S tools into its own infrastructure. It is quite possible that some of the functionality as described in this paper enabling heterogeneous digital libraries to carry conversation might be possible with OWL-S components. Besides these effort, one can imagine building integration between heterogeneous digital libraries systems using existing commercial integration platforms

13 such as for example BizTalk Server , WebSphere Integration Suite 4, Application Server 10g 5 and others. Currently none of these tools supports semantic annotations and none of them allows to mediate between ontology instances. One might attempt to integrate digital libraries with them, but it would be still only one-to-one integration solution mapping syntactical data fields between two different digital library specifications. In the exact field of L2L communication the is a couple of major initiatives that aims to deliver solutions for interoperability between libraries based on different standards of bibliographic descriptions. The OCKHAM initiative 6 promotes development of digital libraries by enabling collaboration between researchers and librarians. Simple, open approaches and standards for digital library tools, services, and content are meant to fill in the gap between classical libraries and emerging digital ones. The OCKHAM Network aims to improve the deployability of the National Science Digital Library (NSDL) in traditional libraries. Another approach, proposed by the Library of Congress 7, is a MARC21 Web Services specification. This web services will be based on MARC 8 standard of bibliographic description.the MARC21 defines a data format that emerged from a Library of Congress initiative that began thirty years ago. It provides the mechanism by which computers exchange, use, and interpret bibliographic information, and its data elements make up the foundation of most library catalogs used today. MARC became USMARC in the 1980s and MARC21 in the late 1990s. As it has been shown, the interoperability in L2L communication can be improved by introducing semantics. The MarcOnt 9 idea of ontology for bibliographic descriptions, is based on the social agreement. The ontology will conform MARC21 together with other bibliographic description standards like Dublin- Core or BibTEX. The use of such an ontology will introduce the Semantic Web technologies to digital libraries world. One of the goals of the MarcOnt initiative is to improve the L2L communication. The idea of open standards for communication is based on web services and Dublin Core as the minimal requirements. The more advanced taxonomy and protocol features can be used as soon as both digital libraries implement them. Attempts to achieve protocol compatibility with existing protocols like Open Archive Initiative are another goals of MarcOnt L2L OCKHAM Initiative: 7 The Library of Congress: 8 MAchine-Readable Cataloguing Standards: 9 MarcOnt Initiative:

14 6 Conclusions and Future Work In this paper we tried to investigate the potential solution to integrate digital libraries. It is clearly seen that semantic enrichment to traditional communication mechanism will enhance quality of service. However, heterogeneity caused by different protocols used in existing digital libraries introduced the problem of interoperability as well as scalability. We identified, that use of WSMX as an intermediary to enable communication between such heterogeneous digital libraries can address the problem of interoperability. Those WSMXs that come between digital libraries when connected to each other using scalable semantic P2P protocols like HyperCup will enhance scalability. The next steps of this research might exploit the dynamic execution semantic mechanism, which is going to be available in WSMX in the future. While currently in WSMX there is a fixed number of components, with predefined interfaces of components with fixed functionality and there is only predefined way how components interacts, this is going to be enhanced with dynamic execution semantics. The dynamic execution semantic mechanism enables any new components to be added easily and any new formal definition of execution semantics to be loaded to the system. Through this paper we shows how existing infrastructure of execution environment for Semantic Web Services improves searching across heterogeneous digital libraries. Having dynamic execution semantics will make the whole system more configurable and peer-to-peer network based on such systems more flexible, what might improve the quality of proposed solution further. References 1. nutshell.html: The protocol z39.50 in a nutshell. (2004) 2. Davis, J.R., Lagoze, C., Krafft, D.B.: Dienst: Building a production technical report server. In: Advances in Digital Libraries. (1995) Lagoze, C., de Sompel, H.V.: The open archives initiative: building a low-barrier interoperability framework. In: ACM/IEEE Joint Conference on Digital Libraries. (2001) Paepcke, A., Brandriff, R., Janee, G., Larson, R., Ludaescher, B., Melnik, S., Raghavan, S.: Search middleware and the simple digital library interoperability protocol (long version). Technical Report SIDL-WP (2000) 5. Okraszewski, M., Krawczyk, H.: Semantic web services in l2l. In Klopotek, Wierzchon, T., ed.: Intelligent Information Processing and Web Mining, Polish Academy of Science, Springer (2004) Proceedings of the International IIS: IIPWM 04 Conference held in Zakopane, Poland, May 17-20, Lausen, H., Roman, D., (editors), U.K.: Web service modeling ontology - standard (WSMO-Standard). Working draft, Digital Enterprise Research Institute (DERI) (2004) 7. de Bruijn, J.: The wsml specification (WSML). Working draft, Digital Enterprise Research Institute (DERI) (2004)

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